1. Field of the Invention
The present invention relates to the field of door latch and lock mechanisms.
2. Prior Art
External door locks are common targets of abuse and vandalism. In particular, latch mechanisms sometimes partially jam due to such reasons as the misalignment between the latch bolt and the striker plate, and warped or partially obstructed doors. Accordingly, in common useage the determination of whether or not a latch mechanism is locked is often made by turning the knob with a torque exceeding that normally required to unlatch the door to see whether the latch mechanism will operate (e.g., is unlocked). Thus, normal useage of a latch mechanism generally subjects the internal parts thereof to substantial loads, separate and apart from any intention to do damage thereto.
In addition vandals, whether under the impression that forcing a knob will force the mechanism to operate, or merely having the intent to damage a latch assembly, may force door knobs into rotation, resulting in the bending or breakage of internal parts in the latch mechanism. Such occurrences are particularly costly as they occur in assembled and installed mechanisms, requiring the removal thereof for replacement on a temporary or permanent basis and/or the repair thereof on an individual basis. These abuses of latch mechanisms, whether intentional or merely minor in every day use, could be substantially eliminated, however, if the exterior door knob on the latch assembly could be made to freely rotate whenever the latch were locked.
In the prior art, typically the exterior latch operating device, whether a knob or a thumb actuator, is directly coupled to the latch mechanism. Accordingly in normal operation the knob or thumb actuator directly actuates the actuator mechanism to withdraw the latch bolt from the striker plate, thereby allowing the door to open. However, when the lock is in the locked condition, the latch bolt mechanism is prevented from operating, thereby directly restricting the movement of the knob or thumb actuator, and creating the opportunity for abuse of the mechanism whether intentional or inadvertent.
One prior art mechanism is known for decoupling the thumb actuator from the latch bolt mechanism when the mechanism is locked. Such a mechanism is disclosed in U.S. Pat. No. 3,933,016 entitled "DOOR LOCK MECHANISM" issued Jan. 13, 1976 and assigned to the same assignee as the present invention and application.